1. Field of the Invention
This invention relates generally to switching amplifiers, and more particularly to a system and method of adaptive pulse frame rate frequency control for minimization of electromagnetic contamination in integrated switching amplifier systems.
2. Description of the Prior Art
Critical frequency band interference is a natural byproduct of switching amplifiers, and is independent of specific architectural approaches and implementations associated with the switching amplifiers. This characteristic is particularly problematic when integrating switching amplifiers into systems with low amplitude front-end-tuners, such as AM/FM/TV band systems. Such integration is presently possible using expensive and bulky metal shielding in association with liberal application of EMI filters on signals, power and ground.
Understanding the key contributors to the EMI spectrum generation accommodates design of a system that can predictably avoid specific frequency spectra. Integrating switching amplifiers into a data communication system generates noise at frame pulse rate, also including the even and odd harmonics of the pulse frame frequency. The noise energy is emitted from the silicon die and is affected by the particular board layout using the switching amplifier(s). The amplitude associated with the noise energy is proportional to the loop areas of the power, signal and ground return areas. In view of the foregoing, it can be appreciated that near field containment of EMI energy due to the contamination of power and ground requires significant design effort. Many EMI filters and shields must, for example, be utilized to ensure that a switching amplifier sub-system can co-exist with a high receptive front end, such as that associated with RF tuner devices. Such tuners however, have selectivity and certain frequency rejection circuitry integrated therein to avoid degradation due to cross-talk (bleed-through) of neighboring stations. Elimination of frequencies generated by a switching amplifier therefore only requires elimination of frequencies which are in the pass band of the tuner""s user-selected frequency. Practically, caution must be exercised to also avoid nearby frequencies to local oscillator frequencies and intermediate frequencies within the IF band generally used in associated receiver technologies.
EMI generally is always present at some amplitude; and frequency spurs associated with such EMI is easily correlated with switching rates and pulse frame rates for communication systems impacted by such EMI. Modern systems passing EMI generally require the system enclosure, input and output signal cables, and AC mains to limit generation of EMI energy in order to conform with specific country or regional EMI standards. Even the most robust EMI compliant systems suffer from sources of self-contamination however, due to near field and board level EMI conducted and radiated emissions.
Brute force methods are useful to reduce the amplitude of EMI noise signals generated by a switching amplifier. These methods, however, add cost and are time consuming to design and optimized. Various manufacturing tolerances must be considered to ensure a robust design for high volume manufacturing and typically add to the system weight, cost and design cycle time. Some of these methods include, but are not limited to 1) use of extensive metal shielding providing a xe2x80x98Faraday cagexe2x80x99 around the emitting source, 2) use of output L-C lowpass fitters, e.g. 2nd order to 6th order, with 4th order being most commonly used, 3) use of high frequency (EMI) filters using ferrite beads, T-filters and the like on power and ground points, and 4) use of EMI filtered connectors to pass all power and signals into and out of the metal Faraday cage.
Spread spectrum switching controller techniques are useful as well to reduce the amplitude of EMI noise signals generated by a switching amplifier. Although such techniques reduce energy in many frequency bands, these techniques often retain sufficient energy in certain critical energy bands and therefore still require use of additional brute force EMI containment devices such as described above.
In view of the above, there is a need for an adaptive frequency programmable pulse frame rate switching amplifier capable of ensuring by design, the elimination of critical interfererce frequency generation. Such a switching amplifier should generate EMI noise signals in certain critical frequency bands only outside frequencies of interest.
The present invention is directed to a switching amplifier that inherently does not produce EMI in certain critical frequency bands. The switching amplifier produces EMI only in frequency bands that are outside the frequency of interest. Two parameters necessary to ensure that interference in a certain frequency band of interest is not generated include 1) the xe2x80x98keep out bandxe2x80x99 for EMI and 2) the frequency range of the switching amplifier necessary to meet acceptable THD, efficiency and frequency response. The xe2x80x98keep out bandxe2x80x99 is known by application and is generally related to the frequency of the AM radio station or FM radio station, for example, that the user would like to listen to or record. Similarly, the xe2x80x98keep out bandxe2x80x99 could be related to a television station that could either be viewed or recorded.
Knowing the frequency of the xe2x80x98keep out bandxe2x80x99 for EMI via user selection as described above, the present system and method of adaptive pulse frame rate frequency control can then be used to change the switching amplifier frequency when the user selects a new frequency band for listening, recording or viewing. This can be done by changing the pulse frame frequency.
Fixed frequency switched amplifiers have predictable frequencies associated with generation of the EMI noise spurs (spikes). Regardless of whether AM, FM, or TV applications are used, there is only a small frequency band with carrier and modulated frequencies (information content) for a given xe2x80x98keep out bandxe2x80x99. These applications can all be accommodated with a system in which the pulse-frame frequency need only change by less than a factor of two. Optimization of normal output L-C filters can therefore remain fixed, even though the pulse frame frequency may be changed to a new value dependent on user selection of the AM/FM/TV frequency.
More specifically, the present invention is an adaptive pulse frame rate frequency control system that receives user selected frequency information such as AM, FM, or TV signals. The control system incorporates at least one switching amplifier and also includes a look-up table or algorithm to determine a proper pulse frame frequency necessary to eliminate critical frequency band interference by the at least one switching amplifier. According to one embodiment, output control data bits for proper pulse-frame frequency selection are decoded and used to control a triangle waveform generator frequency.
In one aspect of the invention, a method and associated system are implemented to eliminate critical frequency band interference by a switching amplifier such that overall channel performance is not limited as a result of small changes in system frequency response and system dynamic range due to changing the pulse frame frequency. Generally, actual information being received by the present adaptive pulse frame rate frequency control system will already be limited in both bandwidth and dynamic range from the source, e.g. station, cable.
In still another aspect of the invention, an adaptive pulse frame rate frequency control system is implemented in which there is not a need to have a unique pulse-frame frequency for each AM/FM/TV station selected due to the actual number of xe2x80x98keep out bandxe2x80x99 frequencies and mathematical relationships (ratios) of those to one another.
In yet another aspect of the invention, a method and associated structure are implemented to eliminate critical frequency band interference by a switching amplifier in which pulse-frame frequencies for each AM/FM/TV station are mapped to specific user-selected AM/FM/TV stations via a look-up table to determine which pulse frame to implement.
Still another aspect of the invention is associated with a system and method implemented to eliminate critical frequency band interference by a switching amplifier in which pulse-frame frequencies for each AM/FM/TV station are mapped to specific user-selected AM/FM/TV stations via algorithmic calculations to determine which pulse frame to implement.